Provide build artifact assembled with JDK8

[martin-gaievski]

Is your feature request related to a problem? Please describe

If OpenSearch plugin has a dependency on classes from main repository this set the lower supported JDK version for the plugin to be same as JDK that used to build the plugin. This causes a problem as OpenSearch declares support for JDK8, but is built with JDK11. If I want to build my plugin with JDK8 I cannot do it, in build time I'm getting errors similar to one below:

java.lang.UnsupportedClassVersionError: org/opensearch/gradle/plugin/PluginBuildPlugin has been compiled by a more recent version of the Java Runtime (class file version 55.0), this version of the Java Runtime only recognizes class file versions up to 52.0

Describe the solution you'd like

Build OpenSearch with multiple JDK and provide ability to set a dependency on a build artifact for each JDK separately (or at least for JDK8 in this case).

Describe alternatives you've considered

Officially declare that the lowest supported version of JDK for plugins may be different from one for OpenSearch itself.

Additional context

Problem arises for kNN plugin in a context of recent changes for 1.3.0 release.

[dblock]

Moving this to OpenSearch. I had started on this in #2300 but I am not sure it's worth the effort as we plan to deprecate Java 8 in 2.0.